This invention relates to electro-optical devices for monitoring the attitude or level of a surface in reference to a reference plane.
According to an aspect of the present invention, an optical level detector, an apparatus for detecting whether or not a planar surface is deviated from its desired plane is disclosed. This apparatus is particularly useful for detecting positional deviations caused by a vertical translation of the planar surface, including vertical translations caused by any pitch or roll. In the apparatus, a light source and a light detector are positioned so that when the surface is at the desired position, i.e. the position of the reference plane, a beam of light projected by the light source is reflected by the surface and fully registers on the light detector. In response, the light detector generates an output electrical signal that is at its peak value. If the surface deviates from its desired position, the reflected beam of light will not fully register on the light detector and the resulting output signal will be less than the peak value.
Two or more of these apparatuses may be used together to monitor the surface at different locations simultaneously. By monitoring multiple locations of the surface, a macro level deviation of the surface can be detected even when local areas of the surface may be at the reference plane. One application, in which the inventor has successfully implemented the optical level detector of the present invention is in the step-and-repeat tools used in the photolithography process in semiconductor manufacturing. Typical step-and-repeat tools (also known as steppers) hold a reticle for transferring circuit patterns on to wafers during the photolithography process. The stepper tool holds the reticle in a reticle stage and if the reticle is not properly level in the reticle stage, the circuit pattern being transferred on to the wafer will not be in focus. Thus, the optical level detector of the present invention provides the stepper tool to fast and accurately verify that the reticle is properly leveled in the reticle stage.